The objective of this research is to determine by electron microscopy the cell type source of rhabdomyosarcomas induced with nickel subsulfide (Ni3S2) in rats. Rhabdomyosarcoma is a common form of tumor in children. Fused nuclei of mature, multinucleated myofibers to not divide. It is, therefore, important to establish the cell type from which these tumors arise. The most common model is muscle carcinogenesis by intramuscular injection of Ni3S2 in mature rats. One possible source of the tumors are mononucleated cells cleaved off degenerating myofibers. The other possible source are the satellite cells. These cells are normally present in low numbers in man and in many laboratory animal species. They are mononucleated and undifferentiated and divide promptly after experimental injuries. The accurate differentiation of satellite cells from mononucleated cells cleaved off degenerating myofibers requires the use of the electron microscopy are compounded by the following conditions; 1) the low number of satellite cells normally present in adult muscles; 2) early necrotic and inflammatory changes; 3) lack of evidence of waves of cell mitotic activity. Satellite cells are more numerous in young than in mature animals. In the proposed experiment a small (1.2 mg) dose of Ni3S2 will be injected into the rectus abdomini muscle of 70 gm rats. After determining when the early inflammatory changes have subsided, a "promoting" stimulus will be applied, which consists of a slight compression of the muscle area infiltrated earlier with the carcinogen. According to experiments in other laboratories such a treatment elicits prompt proliferation of mononucleated cells from muscle. The time and site relationship between muscle compression and cell mitotic activity are well known and it will be, therefore, possible to schedule tissue sampling in such a way as to determine with the electron microscope the nature of the dividing cells and consequently their role in tumor formation.